Step one: get a squirt gun (one with decent range)
Step two: piss in it until it's full
Step three: wait for your neighbor
Step four: hit him over the head with a bat (this step is optional)
Step five: squirt him with piss
(if you did step four, make sure he is conscious before proceeding with step five)
And remember,
Have fun! ;)

Call the cops, say he was naked and he'll get a sexual predator sticker on him. He shouldn't have fucked with you or your mailbox.
That's a double whammy.
That's fucking with a piece of Federal property. What a dumbass. ^_^